Universal joint
 

Looking to fix a rhythmic vibration that occurs only when on the highway.
It is not being felt in the steering wheel, so I plan to start by replacing the universal joint (1981 factory original ) and the rear wheel bearings ( replaced in 1997 ). Already replaced rear tires.

What should I be looking for when purchasing a new Universal joint? Recommended brand, excessive play etc.

Spicer is good. a driveshaft rebalance will take that out of the vibration equation to.

Just a suggestion that I've used as a mechanic hunting vibrations, kick the transmission into neutral when you feel the vibration to see if it changes it at all. Sometimes it's difficult to isolate a vibration from driveline, or engine. Letting the engine drop back to idle will further isolate the source. If it's felt in the steering wheel, it's normally in the front end, if it's felt everywhere it will usually be in the drivetrain.

A bent driveshaft will still vibrate with new joints. I would get the rear wheels off the ground, and observe the driveshaft for runout while turning in gear. Or have it checked by a competent driveshaft shop. It's not that hard to bend an old driveshaft after it's been exposed to corrosive salt, and water for 50 years.

Many times a 50 year old driveshaft will simply have its Welded on balance weight rot off.
Pretty easy most times to still see where it once was .

Right now I have the engine out for the winter.
The vibration does lessen when I let off the gas and coast, but I have not tried shifting to neutral. It only appears at around 60mph and above.
It sounds/feels like rolling over a rumble strip every second or so.
The balance weight is still on the driveshaft, but having it checked is a good idea.

A Bent rim or an unbelted tire can complicates these evaluations, but the neutral at 60 is a darn good start.

My 4L80 project included a cheap heavy driveshaft, so the early drives vibrated, did nothing yet the vibes are totally gone now. super smooth. Driver rear tire was/is suspect due to a side belt wrinkle, yet vibe completely gone.

Does the vibration amplify if you very gently apply the brakes? On the "new doesn't mean good", approach. Increase the rear tire pressure to 50 PSI and take a short drive. See if the vibration changes in sound, intensity, or speed it comes in.

81 TA
 

If thats the same Trans Am as your pic with the warbird wow, thats the nicest 81 I think iv'e ever seen. and I noticed the nice engine specs and power it must be making.

engine out for winter may be a good time to beef up that driveshaft with that kind of power.

heres my original steel 73 firebird driveshaft I had cut to remove the weaker original 1310 u joints and had 1350 series welded on with a new forged sonnax yoke and all rebalanced.

depending on your differential u may need to change the yoke there out to 1350 also.

those vibrations can make u un nerved for sure. specially showing up at 60+ mph

My 65 Lemans had a bent housing tube. That I thought was a driveshaft. Took car to a long curve down hill road. Pulled shaft and coasted down hit about 55 mph. It was the housing.

I doubt it is the rim...I have owned the car since 1992, and I take very good care of them.
It has not seen any winter driving, so no rust on the driveshaft.
Rear tires are new, but there is always the possibility one is not balanced correctly.

I am planning to up the power with a bigger cam than the 60919. I have a CC 290B-6 solid cam on the shelf, just not sure how compatible it will be without more gear.
Also have an old Isky 280 Mega HL cam, but not sure if that is the best choice compared to the more modern grinds.

That's a good suggestion about the yoke. After 40+ years it may be time to give it a closer look.